JFET and Potentiometer questions

[shawnc]

Question 1-Adjusting voltage for JFET
My schematic calls for a JFET to be run at 4.5volts.

How do I measure this, in other words which settings do I use on my mutimeter, where do I connect it to the circuit. Note the circuit uses 100k trim pot.


Question 2-How do I check a JFET's pins for volatge etc..
I have a bunch of different N channel Jfets, and don't have the cash to order any J201's, how can I check which of my other N channel Jfets are close to the J 201, I have about 6 different types and find that the one I am currently using in a 18 watt marshall pedal schem doesn't have very much gain.


Question 3-Potentiometer question
If I don't have any 500meg pots but do have 300meg pots, anyway I can just swapp them in my schematic? What would be the result for example doing that to a volume and gain pot, does it mean less of a taper, or less output, less gain? Do I need to change resistor values to compensate?

Thanks for the help

Cheers

Shawn C

[davebungo]

1) Set your DMM to DC Volts 10 or 20V range.  Connect black lead to gnd, red lead to FET drain and adjust until you see the voltage you need.
2) The best way is firstly to get pdfs of the datasheets and have a look for those which may work.  Otherwise, tell the forum what you have as I'm sure others will have experimented with other devices.  You can try measuring IDSS and VP and so on but it's better to do so once you know you are using a device which may work given the normal spread.
3) I would have thought 300K (surely not 300Meg) would be fine in place of a 500K.  Without seeing the circuit I can't say whether or not you would have to adjust any other resistors, if there are any, to adjust taper etc.

[petemoore]

1) Set your DMM to DC Volts 10 or 20V range.  Connect black lead to gnd, red lead to FET drain and adjust until you see the voltage you need.
adding a testclip to the black DMM lead makes establishing a consistant ground connection easier..
2) The best way is firstly to get pdfs of the datasheets and have a look for those which may work.  Otherwise, tell the forum what you have as I'm sure others will have experimented with other devices.  You can try measuring IDSS and VP and so on but it's better to do so once you know you are using a device which may work given the normal spread.
  >>Jfets of a part # like J201's will vary, as will from one type to another, MPF102's are low gain, 2n5457's, J201's, NTE458's are the highest gain Jfets I've tried .. I stick with the 5457's or the J201's.
3) I would have thought 300K (surely not 300Meg) would be fine in place of a 500K.  Without seeing the circuit I can't say whether or not you would have to adjust any other resistors, if there are any, to adjust taper etc.
  300k to 500k...you can probably adjust slightly the 300k's to provide approximately the same sound as the 500k, if the 'gain' knob is wired as a volume, but in the middle of the circuit, the 500k will shunt just a touch less to ground, minimal difference I'd guess, IIRC theres a little bleed cap maybe, probably don't even need to adjust that caps value....I'd just build with the 300k's, they're easy enough to diddle with [add resistors to to get what you want] or just change to the 500k's if so inclined, I think they'll do just fine...depending on how they're used...
  'Anything RG is a good read, 'What to do when it doesn't work is a great place to start, that or GEO', all the^ links provide info's you'll want to be having I think.

[shawnc]

Lol sorry guys inhaled too much solder, ment to say 500k and 300k potentiometers lol.

Thansk for all the help, very much appreciated guys